Traditional three way catalytic (TWC) converters used in automotive exhaust abatement schemes are typically made by multiple wash-coating cordierite honeycomb monoliths with combinations of aluminum oxide, oxygen storage component (OSC) oxides such as cerium-doped zirconium oxide, and platinum group metals (PGMs).
Of considerable recent interest is a new type of composite material with specific compositions consisting of about 40-70 wt % Al2O3 with the balance typically being made up of CeO2, ZrO2, and perhaps some stabilizers (i.e., the OS component). Furthermore, for these types of composite materials, it is desirable to have the OS and Al2O3 components mixed as homogeneous as possible, and it is because of this that the manner in which the first steps in making such materials, steps involving the co-precipitation of a composite hydroxide and its subsequent treatment, become critical to defining the final properties of the material.
In a typical co-precipitation approach to making a composite material ultimately containing CeO2, ZrO2, and Al2O3, for example, one might first take a fixed amount of an acidic aqueous metal salt solution containing cerium nitrate, zirconyl nitrate and aluminum nitrate and slowly add an alkaline aqueous solution of, for example, sodium hydroxide thereby increasing the metal salt solution pH causing the precipitation of a mixture of cerium hydroxide, zirconium hydroxide and aluminum hydroxide. Since these hydroxides form at different pH values during this acid-into-base co-precipitation, the resulting co-precipitate in the end will not typically be as homogeneous as the same composition made by a base-into-acid co-precipitation, where the pH is always sufficiently high to cause all the hydroxides to simultaneously precipitate.
In a base-into-acid co-precipitation of composites containing hydroxides of zirconium, aluminum and rare earths, the acidic aqueous metal salt solution is slowly added to a fixed amount of an alkaline aqueous solution of, for example, sodium hydroxide. The pH should be kept greater than about 9.0 to assure that all the species precipitate out. However, in a base-into-acid co-precipitation of AlOOH-containing materials, such as those discussed here, the pH also needs to be less than about 10.5. At pH values greater than 10.5, one needs to be concerned about the re-dissolution of the formed AlOOH. In order to maintain such a range in pH (9.0<pH<10.5) using a fixed amount of an alkaline aqueous solution, the concentration of composite hydroxide in the ultimate slurry that forms is usually too low and thus provides yields of products that are too low to be of practical use on an industrial scale.
Alternatively, a fixed amount of an acidic aqueous metal salt solution can be rapidly combined with a calculated amount of an alkaline aqueous solution. Although these solutions are mixed rapidly and in the end the final pH will be sufficiently basic to have caused all the metal hydroxides to precipitate out, during the mixing there may still be temporary undesirable fluctuations in pH, which has a detrimental effect on the properties of the resulting material.
These approaches to making such composite materials are by themselves not novel. In U.S. Pat. Nos. 6,150,288 and 6,306,794 by Suzuki, et al. a method of making CeO2—ZrO2—Al2O3 and CeO2-MOx—ZrO2—Al2O3 composite materials by such approaches is described.
The invention as detailed here is different from what is presented by Suzuki, et al. and involves forming a composite hydroxide precipitate slurry using the following steps:                adding an acidic aqueous solution containing appropriate metal salts in a controlled manner to an alkaline aqueous solution in a reaction vessel with agitation at a specific precipitation pH that is sufficiently basic to cause the formation of the hydroxide precipitate        maintaining a constant precipitation pH during the addition of the acid aqueous metal salt solution by adding a second stream of alkaline aqueous solution in a controlled manner to the reaction vessel until all the acidic aqueous metal salt solution has been added and the precipitation reaction is complete, resulting in a hydroxide precipitate slurry.        
It has been surprisingly found that by using the method described above, a significantly homogeneous composite precipitate can be formed. By maintaining better control over the precipitation pH over the entire course of precipitation than the approach typical of that used by Suzuki, et al, this method allows the production of improved composite materials in commercially viable yields.
A further improvement of this current invention over the approach used by Suzuki, et al. (above) and Yao, et al. (below) for making such composite materials is to thermally treat the resulting aqueous slurry at a pH of between 8.0 and 10.5, at a treatment temperature of greater than or equal to 60° C., and at a treatment pressure of greater than or equal to 0.0 barg for a treatment time of greater than or equal to one hour.
The application of a thermal treatment to a precipitated slurry for such composite hydroxides by itself is not novel. In U.S. Pat. No. 5,580,536 by Yao, et al. a method of making 4.99-98.89 wt % CeO2, 1-95 wt % ZrO2, 0.01-20 wt % HfO2, and 0.1-10 wt % of an additional metal oxide, including Al2O3 is provided and includes a thermal treatment at temperatures of 100-135° C. for 0.5-1.0 hours. However, in the patent by Yao, et al. no mention is made of a specific pH for the hydrothermal treatment. Thermal treatment at the elevated pH values described in the present invention compared to treatments done under closer to neutral pH conditions produce materials after washing, drying, and calcining that exhibit significantly greater aged surface areas, for example. Furthermore, the compositions presented in the current invention are significantly different from the range specified by Yao, et al.
It can often be the case that the sulphate anion and chloride anion levels in composite materials can be too high for automotive catalyst applications. This can even be the case where these anions are not deliberately added during processing. The present invention seeks to resolve this problem by adjusting the pH of the hydroxide precipitate slurry so that it is sufficiently basic to remove these and other anionic contaminants.